If you're a serious collector of books, or are continually looking for inventory to sell online, you should really bookmark this page. I will be compiling a list of sites that will lead you to tons of great places to find antique and collectible books. If you know of any sites that you have found helpful and would like to see added, please email the administrator of this site and we'll see about putting them up here.
Auctionzip.com -This is the best tool I have seen for finding local estate auctions all over the U.S. You can search by zip code and find all the auction houses they have listed within a speciifed number of miles. The auction houses have links to their websites and most post pictures of the items they have for sale at upcoming auctions, so you'll know which ones have antique books.
B2GFree This site is a resource for finding government opportunities and RFPs, etc. The reason I include this site on this page is that it contains one of the greatest resources for finding auctions and sales locally. When you go to the site, look on the left hand side and click on newspapers. You will be taken to a page that lists almost every major newspaper in the U.S. by state, so if you are going to be in an area, you can simply check out the newspaper for that area and locate any auctions or sales that may have collectible books. (You can also navigate around and find state, local and federal government sales which also sometimes include older books.)
Booksale Finder This is an incredible resource for finding large book sales all over the U.S. Many libraries and nonprofit organizations hold large book sales a couple times a year with donated books and retired library books. They are usually one or two day sales with thousands of books for anywhere from 50 cents to 5.00 each. Many bargains can be had at these sales.
Americana Exchange Very intensive site with lots of free information, as well as paid subscriber services. Search a directory of booksellers, find book fairs and auctions and participate in collecting message boards.